1. Field of the Invention
The invention is related to the field of optical communication networks, and in particular, to systems and methods for amplifying optical signals in the 850 nm window.
2. Statement of the Problem
The evolution of fiber optics has provided multiple communication windows for optical fibers. The initial communication window was the 850 nm window that was first recognized as a low loss window. Network engineers then discovered a 1310 nm window and a 1550 nm window that are lower loss than the 850 nm window. Therefore, network design and network component design in the fiber optics area have been focused on the 1310 nm window and the 1550 nm window.
As optical signals travel over a length of optical fiber, the optical signals experience some degradation due to attenuation along the fiber. After traveling over 10 km or more of fiber, amplifiers are needed to recover the signal strength of the optical signals. The 1550 nm window and the 1310 nm window each have rare-earth doped optical amplifiers that can recover signal strength. For the 1550 nm window, an Er-Doped Fiber Amplifier (EDFA) is used to amplify the optical signals. For the 1310 nm window, a Pr-Doped Fiber Amplifier (PDFA) is used to amplify the optical signals. Unfortunately, there are no suitable rare-earth doped optical amplifiers for the 850 nm window.
An article by S. P. Dye, M. Fake, and T. J. Simmons entitled “Fully engineered 800-nm Thulium-doped fluoride-fiber amplifier” (referred to herein as the Dye article) describes an optical amplifier in the 800 nm range. The amplifier includes a span of Thulium-doped Fluoride fiber and two pump lasers on single wavelength. One of the pump lasers backward pumps the Thulium-doped Fluoride fiber at 780 nm, and the other pump laser forward pumps the Thulium-doped Fluoride fiber at 780 nm. The Dye article does not provide what gain this amplifier configuration actually produces, but does state that the configuration may provide useful amplification levels between 795 nm and 820 nm. The 850 nm window is commonly known to be between 830 nm and 860 nm. Therefore, the configuration in the Dye article does not amplify signals in the 850 nm window, and does not solve the problem of an amplifier for the 850 nm window.
Also, the configuration in the Dye article is optimized to amplify optical signals around 1480 nm, and the amplification of optical signals between 795 nm and 820 nm is a secondary consequence of the dual pumping at 780 nm. Thulium-doped fiber amplifiers (TDFA) are commonly used for gain in the S-band. Gain is also generated in the 800 nm region according to the configuration in the Dye article. The gain at 1480 nm would be between 25 dB and 30 dB, while the gain in the range of 795 nm to 820 nm would only be about 3 dB. This low of a gain in the range of 795 nm and 820 nm would not provide any useful amplification as desired by network engineers.
Thus, a problem remains to provide adequate amplification for the 850 nm window.
Because of the lack of amplifiers, the 850 nm window is used only for short reach applications that are less than about 10 km. The recent development of photonic crystal fibers, which have zero-dispersion wavelength around 850 nm, makes it desirable to solve the problem of amplification in the 850 nm window so that the 850 nm window may be used for other applications, such as long haul applications.